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Join Date: May 2010
Location: Cambridge, MA
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Re: Voltage vs. Current

04/20/2011 2:13 PM

Yes, 2krishnanunnis, BJTs are transconductance devices (voltage in, current out), just like tubes and FETs, which means they are first and foremost, voltage controlled.

To answer your question: How to tell?, I'd say the physics and formulas are the key. In the case of the transistor we have the solid-state physics resulting in the rigorous Ebers-Moll formulas, with their precise prediction for collector current IC from VBE, namely IC = IS (eVBE/VT -1), as against the vague squishy β or HFE gain, if we try to use current for biasing, etc. Just because you can successfully bias a few BJT circuits with current doesn't meant they're current-controlled devices.

Is a long-tail pair differential-amplifier going to work with current control to the two base inputs? No way. Is a cascode-connected stage going to work with current control to the base? No, of course not, it can't. Is an emitter-follower driving a common-mode amplifier going to work with current control to the emitter-follower's base? No. How about the operation of a current mirror? Hah!

The gain of common-emitter stage is RL/re, where re = k T / q IC, in a tightly-controlled fashion. re = 25 ohms at 1mA, and scaling inversely to current, 250 ohms at 100uA, 2.5 ohms at 10mA. Very useful to remember. Where is base current in this scene? How does it help you determine the gain of a common-emitter amplifier? Nada.

Thanks, Win
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